The present invention relates in general to semiconductor devices. More specifically, the present invention relates to trench structures, which can be used to enhance the performance of semiconductor devices.
There exist a variety of semiconductor devices commonly used in power applications. One such device is the Schottky barrier. A Schottky barrier comprises a metal-semiconductor interface, which functions as rectifier for controlling current transport.
A figure of merit, which is used to measure the blocking capability of a Schottky barrier rectifier is its breakdown voltage. A breakdown voltage in this context refers to the maximum reverse voltage, which can be supported across the device, while still being able to provide a blocking function. Breakdown in a Schottky barrier rectifier is normally an xe2x80x9cavalanchexe2x80x9d type breakdown, which is predominantly attributable to a phenomenon known as xe2x80x9cimpact ionizationxe2x80x9d.
FIG. 1 shows a cross section of a basic Schottky barrier rectifier 10. A first metal layer 100 is formed on a semiconductor layer 102. Typically, the semiconductor layer 102 is comprised of an epitaxial layer 104, which lends itself as a drift region, and a more heavily doped substrate 106. Heavily doped substrate 106 and a second metal layer 108 provide an ohmic contact for the device.
Applying a reverse bias voltage VREV across the Schottky barrier rectifier 10 creates a depletion region 110, across which a majority of the applied voltage is dropped. As the reverse voltage is increased, electric fields in the depletion region 110 become greater. These increasing electric fields cause the charge carriers to accelerate and, if sufficiently accelerated, can cause the creation of electron-hole pairs by collision with dopant atoms. The more carriers that are generated, the more carriers having sufficient energy to cause impact ionization there become. Hence, impact ionization is a snowball effect, whereby a cascade of electron-hole pairs are created by a succession and multiplication of collisions. A point is eventually reached where the rate of impact ionization is so great that the device cannot support any further reverse bias applied across it. This voltage limit is commonly referred to in the art as the xe2x80x9cavalanche breakdown voltagexe2x80x9d.
The basic Schottky barrier rectifier 10 shown in FIG. 1 is limited by its reverse blocking capability, since electric fields tend to converge at the edges of the metal layer 100. Because of this, techniques for terminating the Schottky barrier rectifier have been sought. Two commonly used techniques, which reduce the edge effects are a local oxidation of silicon (LOCOS) structure and the diffused field ring structure described in xe2x80x9cModern Power Devicesxe2x80x9d by B. J. Baglia, 1987, Reprinted Edition, pp. 437-438. These two approaches are shown here in FIGS. 2 and 3. Each of these prior art techniques has the effect of reducing electric field crowding at the metal edges and, consequently, a higher breakdown voltage is achieved.
A technique proposed to achieve even better reverse blocking capabilities in a Schottky barrier rectifier is described in Wilamowski, B. M., xe2x80x9cSchottky Diodes with High Breakdown Voltages,xe2x80x9d Solid State Electron., 26, 491-493 (1983). A cross section of the structure proposed in this article, referred to as a Junction Barrier Controlled Schottky Rectifier (i.e. xe2x80x9cJBS rectifierxe2x80x9d), is shown here in FIG. 4. A series of p-type regions 400 are formed in and at the surface of the drift region 402 of the device. These p-type regions 400 act as screens to lower the electric field near the surface. Since electric fields at the surface are what determine the breakdown voltage of the device, introduction of the p-regions 400 results in a higher breakdown voltage.
An undesirable characteristic of the JBS rectifier relates to the p-n junctions, which are formed between the p regions 400 and the drift region 402. For silicon devices having a high reverse breakdown voltage, a forward bias exceeding 0.7 volts is required before a reasonable forward conduction current of the Schottky barrier can be realized. Unfortunately, voltages higher than 0.7 volts have the effect of turning on the p-n junctions. When on, minority carriers are introduced, which slow the switching speed of the device. A reduction in switching speed is undesirable, particularly if the Schottky barrier rectifier is to be used in switching applications such as, for example, switch-mode power supplies.
To overcome the forward bias limitations associated with the JBS rectifier, an alternative device structure has been proposed, which utilizes a series of parallel metal oxide semiconductor (MOS) trenches in replace of the p-type regions. This MOS Barrier Schottky Rectifier (i.e. xe2x80x9cMBS rectifierxe2x80x9d) is proposed in B. J. Baliga, xe2x80x9cNew Concepts in Power Rectifiers,xe2x80x9d Proceedings of the Third International Workshop on the Physics of Semiconductor Devices, November 24-28, World Scientific Publ. Singapore, 1985. A cross-section of an MBS rectifier 50 is shown in FIG. 5A. It comprises a first metal layer 508, over which a semiconductor layer 502 is formed. Typically, the semiconductor layer 502 is comprised of an epitaxial layer 504, which lends itself as a drift region, and a more heavily doped substrate 506. Heavily doped substrate 506 and first metal layer 508 provide an ohmic contact for the device. MBS rectifier 50 also includes a number of parallel trenches 512 formed in epitaxial layer 504, each of which has an end that terminates (or xe2x80x9cmergesxe2x80x9d) with a termination trench 514, which includes a segment that runs essentially perpendicular to the parallel trenches 512. Termination trench 514 and parallel trenches 512 are lined with a dielectric 516, e.g. silicon dioxide, and are filled with a conductive material 518, e.g. metal (as shown in FIG. SA) or doped polysilicon. A second metal layer 520 is formed over the entire surface of the structure. Note that in FIG. 5A, metal layer 520 is shown as only partially covering the surface. However, this is done so that underlying elements of the rectifier 50, which would otherwise be covered by metal layer 520, can be seen. The metal/semiconductor barrier of MBS rectifier 50 is formed at the junction between second metal layer 520 and upper surfaces of mesas 522 formed between parallel trenches 512.
In many respects, the MBS rectifier is superior to the JBS rectifier. However, it too has limits on its reverse blocking capabilities. These limits can be illustrated by reference to FIG. 5B, which shows a top or xe2x80x9clayoutxe2x80x9d view of the MBS rectifier in FIG. 5A. The arrows, at the ends of mesas 522, which point at labels xe2x80x9cExe2x80x9d (xe2x80x9cExe2x80x9d electric field), are present to show how under reverse bias conditions, electric fields tend to crowd toward the ends of mesas 522. This electric field crowding phenomenon is due to faster depletion in these regions, compared to other regions in semiconductor layer 502. Accordingly, the breakdown voltage of the MBS rectifier shown in FIG. 5A is determined and, therefore, limited by the trench structure geometry illustrated in FIG. 5B.
Generally, a broken trench structure enhances the breakdown characteristics of semiconductor devices, according to various aspects of the present invention. For example, as explained in more detail below, a Schottky barrier rectifier, when integrated with the broken trench aspect of the present invention, shows enhanced reverse blocking capabilities, compared to that achievable in prior art structures.
According to a first aspect of the invention, a MOS trench structure integrated with a semiconductor device for enhancing the breakdown characteristics of the semiconductor device comprises a semiconductor substrate; a plurality of parallel trenches formed in the semiconductor substrate, each parallel trench defined by end walls, sidewalls and a bottom and each two adjacent parallel trenches separated by mesas containing the semiconductor device, said mesas having a mesa width; and a peripheral trench defined by ends, sidewalls and a bottom, said peripheral trench at least partially surrounding the parallel trenches, and said peripheral trench being spaced from the ends of the parallel trenches by a parallel trench to peripheral trench spacing; a dielectric material lining the ends, bottoms and sidewalls of the parallel and peripheral trenches; and a conductive material substantially filling the dielectric-lined trenches.
This aspect of the invention and others, together with a further understanding of the nature and the advantages of the inventions disclosed herein is described now in reference to the remaining portions of the specification and the attached drawings.